In recent years, a metal halide lamp (which will be hereinafter referred to as a discharge lamp) is utilized as a lighting device for a vehicle (a headlamp) in place of a conventional halogen lamp having a filament. The discharge lamp has a higher light emission efficiency and a longer lifetime as compared with the halogen lamp. However, the discharge lamp requires a driving voltage of several tens to several hundreds V. For this reason, the discharge lamp cannot be directly driven by an on-vehicle battery of 12 V (or 24 V) so that a discharge lamp lighting circuit (which is also referred to as a ballast) is required.
A method of turning ON the discharge lamp is classified into DC driving and high frequency driving. When the DC driving is carried out, however, a discharge arc is asymmetrical so that the light emitting profile is not uniform. For this reason, the method is not suitable for a utilization as the lighting device for a vehicle, and AC driving is generally carried out in the lighting device for a vehicle. When the discharge lamp is subjected to the AC driving at a high frequency of 10 kHz or more, a phenomenon occurs in which an air current in a discharge tube and a lighting frequency are resonated (which is referred to as an acoustic resonance). As a result, the discharge arc is unstable. In order to eliminate disadvantages of both the DC driving and the high frequency AC driving, a method of carrying out driving at a low frequency of 10 kHz or less (a low frequency driving method) is a mainstream at present.
The discharge lamp lighting circuit includes a DC/DC converter for raising a battery voltage, a switching circuit such as an H bridge circuit for AC converting an output voltage of the DC/DC converter, an auxiliary lighting circuit and a starter circuit (for example, see Japanese Patent Document JP-A-11-329777).
As disclosed in FIG. 1 of JP-A-11-329777, the auxiliary lighting circuit (which is also referred to as a takeover circuit) is provided in parallel with an output smoothing capacitor of the DC/DC converter and is constituted by an auxiliary lighting capacitor and an auxiliary lighting resistor which are connected in series. At a start of a lighting operation of the discharge lamp, the following sequences are executed.
1. Power ON
2. Breakdown
The DC/DC converter is operated to raise a battery voltage up to approximately 400 volts (V). The voltage of 400 V is further raised to be 20 kV or more by the starter circuit to generate a high voltage pulse and the discharge lamp is broken down to start a discharge.
3. Arc Growth
Immediately after the breakdown, an overcurrent of several amps (A) is supplied to the discharge lamp by using an energy which is pre-stored in the output smoothing capacitor of the DC/DC converter and the capacitor of the auxiliary lighting circuit. Thus, a lighting failure is prevented and, at the same time, a transition from a glow discharge to an arc discharge is carried out.
4. Run-up
When the arc discharge is started, a light output of the discharge lamp is raised. The rise in the light output is determined by standards. In order to obtain a light output (a power) matched with the standards, the discharge lamp lighting circuit monitors a lamp current flowing to the discharge lamp, and a lamp voltage applied to the discharge lamp and regulates a duty ratio of ON/OFF of a switching unit in the DC/DC converter through a feedback. For a run-up period, a higher overpower than a rated power is temporarily supplied to the discharge lamp.
5. Stationary Lighting
Then, the power to be supplied to the discharge lamp is stabilized to have a rated value so that the light output of the discharge lamp is stabilized.
The auxiliary lighting capacitor of the auxiliary lighting circuit serves to store an energy (an electric charge) to be supplied to the discharge lamp in an arc growth. If a capacitance value of the auxiliary lighting circuit is increased significantly, the discharge lamp is turned ON more easily. On the other hand, if the capacitance value of the auxiliary lighting circuit is increased, the following problem is caused in stationary lighting.
More specifically, when the discharge lamp is subjected to AC driving, a direction (polarity) of the lamp current is inverted at a lighting frequency. In a polarity inversion timing, however, the discharge lamp is turned OFF in a moment. In a polarity switching timing, a transient voltage is applied to the discharge lamp by a back electromotive force generated in a high voltage coil (a part of the starter circuit) provided in series to the discharge lamp. Thus, a stable current is caused to flow after the polarity switching (which will be hereinafter referred to as a re-ignition).
When the capacitance value of the auxiliary lighting capacitor is increased, however, the back electromotive force generated by the high voltage coil in the re-ignition is absorbed into the auxiliary lighting capacitor. For this reason, there is a possibility that the re-ignition will be difficult to perform and the discharge lamp might cause a lighting failure. When the capacitance value of the auxiliary lighting capacitor is reduced to prevent the lighting failure, there is a possibility that a transition to the arc discharge will be hindered. Similarly, the problem might be caused in the case in which a resistance value of the auxiliary lighting resistor is small in addition to the case in which the capacitance value of the auxiliary lighting capacitor is great. Furthermore, the problem might be caused also in other discharge lamp lighting circuits in addition to the vehicle discharge lamp lighting circuit.